1. Field of the invention
The present invention relates to a receiver for a mobile radio terminal. The invention therefore relates more particularly to telecommunication systems.
2. Description of the prior art
Analog components in the radio frequency portion of a mobile radio terminal degrade the received signal supplied by the antenna.
This applies to the local oscillator in particular. The local oscillator produces a radio frequency signal that is fed to the frequency changer at the same time as the received signal to eliminate the carrier on which the received signal is transmitted on the propagation channel.
The frequency changer transposes the frequency of the received signal to a lower frequency.
The local oscillator and the frequency changer have inherent defects.
In particular, the local oscillator adds a direct current (DC) component to the received signal, whose frequency is therefore 0 Hz (Hertz). This DC component shifts the received signal upward or downward and its amplitude depends on the integrated circuit and on the isolation of the card.
Leaks from the local oscillator are radiated into the atmosphere after finding their way back to the antenna of the mobile radio terminal and reach the receiver of the mobile radio terminal after being reflected from external obstacles, thus creating an unwanted signal in addition to the received signal. This unwanted signal constitutes the dynamic component of the interference caused to the received signal by the behavior of the local oscillator and the frequency changer.
The frequency of the line that occurs due to the to-and-fro phenomenon described above cannot be controlled, in particular because of the time at which the reflection occurs and the speed at which the mobile radio terminal is moving. The frequency of the dynamic component is therefore related to the Doppler effect and depends on the speed of the mobile radio terminal.
The amplitude of the dynamic component cannot be controlled either.
The operation of the local oscillator and the frequency changer in the receiver of the mobile radio terminal therefore causes interference in the received signal. This interference is reflected in the appearance of a DC or static component at a frequency of 0 Hz and a dynamic component which can have any frequency and amplitude.
The static and dynamic component must be eliminated if the receiver of the mobile radio terminal is to operate correctly.
One prior art solution provides the receiver of the terminal with a filter to eliminate the static component and the dynamic component of the interference to the received signal induced by the local oscillator and the frequency changer.
The filter used is a high-pass filter. It is inserted between the frequency changer and the analog-to-digital converter in the receiver of the mobile radio terminal. This solution is therefore implemented in the analog domain.
However, using this kind of filter in the receiver has serious drawbacks, as shown in FIG. 1.
FIG. 1 shows the 0 Hz static component 1, the dynamic component 2, which has a frequency equal to fd, the spectral response 3 of the high-pass filter used, and the wanted signal 4, i.e. the received signal carrying the required information.
The filter is characterized by a slope and a cut-off frequency fc. The cut-off frequency fc is chosen to be sufficiently high to accommodate the maximum shift of the dynamic component 2, so that fc=fdmax. The frequency of the dynamic component 2, which depends partly on the speed at which the mobile radio terminal is moving, as explained above, cannot be controlled and changes as a function of the Doppler frequency. Accordingly, the high-pass filter used is characterized so that the receiver can accommodate a particular range of speeds and therefore high variations in the dynamic component, extending up as far as that range of speeds.
The filter is fixed and cannot be adapted to all situations.
Accordingly, in the FIG. 1 example, although the filter used is able to eliminate the static and dynamic components, it also strongly attenuates the whole of that portion of the modulated wanted signal denoted a in FIG. 1.
The unwanted attenuation of the portion a of the wanted signal seriously degrades the signal. This in turn degrades demodulation performance, which is reflected in a degraded bit error rate (BER), and the reduction in performance is directly related to the characteristics of the high-pass filter used, in particular its cut-off frequency.
A second prior art solution eliminates the static and dynamic components of the interference to the received signal by processing the signal following analog-to-digital conversion using a Least Mean Square (LMS) algorithm. The received signal is then processed entirely in the digital domain.
However, this type of algorithm is very complex to implement and necessitates a very high computing power, which can be unacceptable, and also means that the analog-to-digital converter cannot be optimized.
Accordingly, the object of the present invention is to eliminate both the static component and the dynamic component of the interference to the received signal induced by the operation of the local oscillator and the frequency changer in a manner that alleviates the drawbacks of the prior art, i.e. without degrading the demodulation performance of the mobile radio terminal and without unreasonable complexity.
To this end, the invention proposes to combine the two solutions previously described.
In accordance with the invention, the processing of the signal to eliminate the static and dynamic components of the interference is divided into two portions.
A first portion is effected in the analog domain and the second portion is effected in the digital domain.
Thus elimination of the static and dynamic components is shared between the analog domain and the digital domain.